In a transponder a radio frequency signal is transmitted to a transponder, which in turn retransmits the signal, often in modulated form, that is to say with superimposed information from the transponder. The purpose of a transponder may thereby be partly to act as a signal repeater, partly exchanging information with the transponder. Some transponders work indirectly, others directly. In indirect retransmission, the signal is received and retransmitted in sequence. Retransmission may be desired to take place in a frequency band different from the band for received signal. One example is aircraft transponders for DME. In direct retransmission the signal is transmitted simultaneously as it is received, in the same band. Here, the conversion- and modulation-gain of the transponder is utilised. Examples are RFID TAGs. In the lastly mentioned case the transponder acts as an amplifier, often with very small or negative amplification. Such transponders therefore, serve few applications within the various areas of wireless communication and radio navigation.
A transponder is in many cases in addition to retransmission (up link) also required to receive information (down link) to identify itself and act on commands. Applications that use transponders are therefore often named RFID systems (Radio Frequency IDentification). It is frequently required that the transponder is portable, lightweight, compact, simple and carries few components, is inexpensive to manufacture and has several years of battery life, at the same time as available performance margins become inadequate, especially with respect to communication range. At the same time, requirements of large communication bandwidth and multi channel operation are present. It is often required that transponders have coherent retransmission, either with respect to an interrogator or a phase measuring station when the transponder is to be positioned as well.
The most commonly used principle for transponders is the so-called reflective principle. It works with a RF carrier from a beacon or interrogator received by an antenna, which is coupled to a high frequency diode that in turn is modulated by the signal, to be retransmitted to the interrogator by the transponder. Usually the aim is to achieve phase modulation, which is easily accomplished by having a diode switching the refection coefficient in the antenna connection terminals. The resulting modulation will always be a combination of amplitude modulation and phase modulation with no significant performance reduction. The retransmitted (up link) side bands are coherent with the incoming signal, and the interrogator works by the homodyne principle. To avoid cancellation between side bands, single side band reception with side band cancellation is used in the interrogator.
Reception (down link) in transponders is accomplished with the mentioned diode or a dedicated diode demodulating the high frequency signal from the antenna directly, without high frequency amplification. High frequency amplification is not used, mostly on account of power consumption. The sensitivity acquired is therefore limited, but may be well tuned to the transponder dynamics achieved with the reflective principle.
The disadvantage with the reflective principle is that the retransmitted signal level only can be amplified by the help of antenna gain. Too much antenna gain is unwanted, because high antenna gain gives too narrow antenna lobes and consequently pointing errors, and the result may therefore become losses in stead of gain.
In a few existing transponders, active amplification is introduced, that is active high frequency or microwave components, to achieve this. With conventional technology this comes with high costs in the form of high power consumption and costly products. Power consumption becomes high because unconditionally stable amplifiers are required. Cost becomes high because this, on microwave frequencies, usually is accomplished with microstrip technology and expensive circuit board laminates. The amplification achievable is very limited due to current draw and because it is difficult to sustain sufficient isolation between transmitter and receiver in low cost products. This implies that such solutions preferably must carry separate transmitter and receiver antennas. Benefits of such solutions are usually not worth the increased cost, and the majority of such products today therefore have passive microwave modules, that is just one diode or a transistor switch. The solutions are likely to require a limiter which serves to limit the transmitted level below the maximum allowed level according to the respective code or standard for the application of the transponder. Limiter and filter may also be required to achieve necessary suppression of harmonics of the modulation frequency. Harmonics of the RF carrier are often very difficult to suppress sufficiently to meet standard requirements. Transponder range for the transponder solutions mentioned is very limited, because the outgoing signal amplitude is nearly proportional to the incoming signal amplitude as a consequence of no or little active high frequency amplification in the circuit. Such amplifying transponders therefore have seen few applications within the various areas of wireless communication and radio navigation concerned.
Some known systems concerning interrogation of sensors or various types of platforms that require a low current, simple transponder, have effective solutions for down link in the transponder, while the up link is comprised by one or more oscillator functions. A significant disadvantage with this solution is that it will require a crystal oscillator for the transmitter if the purpose is not served by the poor frequency stability and calibration otherwise resulting. Such a transponder is not usable in a homodyne system unless it carries a phase locked loop (PLL) frequency locked to the interrogator.
It has been shown that transponders may be realised as simple, injection locked oscillators. These have specifications that seriously limit their applications. The injection locked oscillator is in principle any type of oscillator circuit where oscillator stability purposely is made dependent on no outside noise or an injected CW signal (see below) which is closely equivalent to the oscillator frequency to give frequency locking. The circuit is compensated for temperature and other types of instabilities. The frequency spectrum of an injection locked oscillator unlocked and with no signal in as well as locked to a signal in, appears as the spectrum of an ordinary oscillator with a CW carrier. With an in signal and out of lock it will have a typical, strong phase noise on one side of the carrier frequency. As mentioned, the largest disadvantage of the injection locked oscillator is a very narrow lock frequency band and a very low sensitivity. The advantage is low phase side band noise. There is a need for a technology, which improves the injection locked oscillator and expands the applications there of. One example of injection locked oscillator application is in phased antenna arrays, but there, as well, the usefulness is limited on account of narrow locking bandwidth which typically will be some ten thousandths of the carrier frequency, and in addition a CW signal is required. (In the following text, the term CW is both used for a RF carrier, which is either continuos or pulsed. This is in line with the conventional literature, although CW carrier actually is supposed to mean “continuous wave”. Physically speaking, a continuous wave cannot exist in reality. “Quenched oscillator” is used meaning an oscillator, which is quenched with a repetitive function with frequencies from kHz to MHz). It has been shown, see U.S. Pat. No. 3,705,385, how an injection locked oscillator may be improved, especially with regard to locking bandwidth, with so called quenching, that is switching of the oscillator. Still, the locking bandwidth is narrow, typically some thousandths of the carrier wave frequency, and still a CW signal is required, often limited to FM modulated CW, to allow signal repetition to work satisfactorily. Besides, the locking is heavily dependent on the signal dynamics and will generally only work for strong CW signals. It appears that one has believed it necessary that the carrier frequency itself had to be locked in order for a number of transponders to work together without interference. That may have been a cause for the super-regenerative principle to have been overlooked for such applications, see below. Another reason is that it is a far more difficult challenge to make a quenched oscillator work according to the intention in superregenerative mode than in injection locked mode, due to added component requirements besides design challenges. This follows from the fact that superregenerative function generally occurs or is effective only in a narrow region of the bias characteristic for the oscillator, while the injection locked function occurs across a large part of the remaining characteristic. This is little or not discussed in publications about SG applications. In addition, the quench frequency is often injected in such a way that the superregenerative dynamic range is severely limited, which again shows how poorly the circuit was analysed. It has not been shown earlier how unwanted emission of signals and inter- and cross modulation products should be reduced in order for a quenched oscillator to work in accordance with standards. Development in component technology has additionally made it possible to utilize the superregenerative principle better, with very low power solutions, to assist innovations using this principle. The quenched, injection locked (=locked) oscillator has, as explained herewith, specifications that iimply large limitations with respect to signal dynamics and bandwidth and further disadvantages like reliability, that reduce possible applications. This is proven by the fact that earlier publicised and patent text technologies having failed to succeed in applications (i.e. see U.S. Pat. No. 3,705,385), which is due to several factors, some of the more important ones being unreliable frequency locking and narrow useful information bandwidth in the kilobaud range. Such bandwidth is mostly rather uninteresting for today's communication technologies. Additionally, it is not evident from subsequent patents and publications if anyone made serious attempts to improve the technology or widen the scope of the use of narrow band, locked oscillator.
There is a need for finding alternative solutions to known transponder technology that uses “on board” oscillator. There is a need for a transponder technology which achieves to combine simplicity with existing, reflective transponders with wide bandwidth, high performance, stability, power efficiency, production applicability and that in addition allows simple and cost effective implementations in microwave ASIC (customer specified integrated circuit) or MMIC (microwave integrated circuit). There is also a significant need for a new technology where the performance of transponders exceed minimum requirements so that margins and production compatibility are increased and to allow microwave transponder systems to be realised with less expensive substrate technologies and without the use of micro strip.
Common uses of transponders are sensor systems, control systems, medicine and in RFID systems. An example of the use of sensor systems is the need to improve existing technology for surveillance, control and communication in power distribution in high and low voltage power line distribution systems. An example of control systems is measuring and actuating tasks in processes, both in- and out-door. An example of medical usage is the application of sonds and sensors in medical scientific research. An example of RFID usage is given by the need for identifying and communicating with objects, persons and vehicles on long range. One application for simple transponders in RFID which include long range, is radio tagging of animals, where limited range for today's transponders makes them less suitable and therefore other technologies are used like pulsed beacons that renders less service per carried energy unit because continuos transmission is required. Long range may be defined as from ten meters to several kilometers. One widespread application with RFID is intelligent and unintelligent “tags” for identification, access pricing and payment etc. Transponders for different application areas are most likely to use frequencies between 30 MHz to over 10 GHz. In toll road systems and similar, microwave bands 2.45 GHz and 5.8 GHz and more are used.
Nodes in some signaling networks or data communication networks may be regarded as indirect repeaters. Examples of such are cellular phones, or mobile systems (i.e. GSM, GPRS, UMTS, TETRA). If nodes or stations in such systems are to be used for retransmission, it leads to a significant reduction in information bandwidth, usually reduces to half. The same applies to nodes in Wireless LAN, Bluetooth and other wireless data communication networks. This seems to be the reason for repeating functions usually not being implemented in the mentioned systems. There is a significant need for a new system, which is compatible with existing, and future, wireless network and communication systems and which is able to repeat signals in both directions. There is also a need for inexpensive and effective technology in nodes for such networks, which is also able to perform repeating functions without reducing bandwidth from the repeating function. In some cases there will be needs for the transponders to act intelligently.
The evolution of radio based, wireless networks for large bandwidths that is required to use very high frequencies (10-200 GHz) have been hampered by the fact that it is still too expensive to implement transmitters, receivers and transceivers. Up to now, it has not been possible to realise a simple transponder with large dynamics for such frequencies. At the same time, there is a need for implementation of inexpensive, local wireless networks with large bandwidths of more than 100 Mbit/s. There is a great need for a system technology, which allows inexpensive networks in the cm and mm wavelengths.
In wire and cable based communication systems the same as for wireless systems is valid. Line amplifiers are expensive to realise and often they can amplify the signal in one direction only. Examples of line amplifiers that are bi-directional are older type amplifiers for phone lines that exhibit low amplification and can only be used for low frequencies. Examples of the amplifiers that have high amplification, but are unidirectional are cable TV amplifiers used for data communication. For high frequency it has been possible to make line amplifiers with limited isolation between the amplifier input and output, with resulting low useful amplification and therefore applications are very limited. It is therefore a need for a new principle of amplification of signals along a signal cable with the help of simple methods implying small or no modifications to the system.
Within positioning, radio navigation and distance measurements, coherency and controlled phase relationships are desired parameters. An example is hyperbolic positioning systems where the phase of the measured signal must be determined by clock regeneration. This puts strict requirements on real time processing and filtering, and often reduces the update rate of the system. In many positioning systems for short and medium ranges there is a need for a transponder technology which will work effectively and which retransmits signals with known phase. Applications would be in objects to be positioned or as parts of known infrastructure of the system to improve the measurement geometry of the system. Until now, such transponders have been too expensive to make or have not been realisable. There is also a need for an inexpensive, low power and effective transponder technology, which may increase the usage of radio positioning by positioning people, belongings a.s.o. For recovering purposes there is also a need for an inexpensive and more efficient and useful technology for transponders.
In power line surveillance and communication there used to be a need for series connected amplifiers (line amplifiers) in the lines or cables to compensate for signal losses. This has been excessively expensive and may cost tens of thousands of US dollars per connected unit. It follows that there can be only a small number of amplifiers along the lines, resulting in a very low communication bandwidth. Likewise, it is expensive and complicated to bypass transformers and other infrastructure in the power network for communication signals. It exists therefore a need for a new principle of amplification of signals along the power line network with the aid of simple methods that require minor or no modifications of existing installations, and which makes it possible to realise far wider transmission bandwidths and better flexibility. With known technology it is not possible to have distributed surveillance along a power line and existing solutions therefore use expensive, widely spaced installations that use radio communication. There if therefore a need for a new technology which integrates all types of surveillance and control in any position in the power line network, with two way communication along the power lines.
In power line surveillance and communication on the distribution circuits, where data communication is to include so called access networks for broad band distribution and other communication wtih clients, the communication range will be limited to 100 to 300 meters due to signal losses. Line amplifiers are very expensive to realise and install and indirect repeaters reduce the data bandwidth. Consequently it is often difficult to transmit signals between clients and other units like routers, masters and hubs. With known technology there exists no solution, which in a simple and inexpensive way can relay signals without galvanic coupling passed embedded separations in a power network, i.e. a transformer station. It exists therefore a need for a new principle for amplification of signals in electricity networks used as access data networks employing simple methods requiring small or no modification of the infrastructure.
In communication systems of different kinds, local shadow zones will easily occur. This is particularly true in mobile communications as with GSM, GPRS, UMTS, TETRA and more. Here, it has until now been impractical to realise inexpensive transponders or repeater systems to amplify the signals in a simple manner and in such a way fill in coverage holes or shadow zones. Known technology did not achieve the necessary signal amplification and one therefore was obliged to install an additional base station to serve a coverage hole area. Such insufficient coverage therefore had to be accepted as along roads, within buildings, ships, ferries and so on. Power lines are found along roads that could function as carriers or hosts for small transponders and could also power them as an example with inductive transmission of the moderate energy required. With known technology, it is neither easy nor cost effective to couple shielding rooms in buildings, ships a.s.o. to the outside world to achieve radio coverage. By this reason, there is a need for a new principle of amplification of signals in systems for mobile communications with the help of simple and inexpensive methods that requires little energy. Correspondingly, there is a great need of a new technology that allows simple signal repeating or signal amplification for radio applications within systems and equipment for broadcasting and communication. This is particularly true for local, geographic are as. In other communication systems where passive RF technology or low transmitting power is used, like in RFID tags, the margins often are small giving communication problems from changing conditions of various kinds. There is a significant need for an inexpensive, energy efficient transponder technology that easily can amplify signals in both directions that as an example could be installed on r near such a low power device. In this case, it seems logical to name the transponder a “signal booster”. In optical signal transmission systems there may be a need as well for a new technology that in the same manner as the super regenerative principle with radio waves and by loose coupling to an optical waveguide or other optical medium, amplifies the signals.